Noble metal catalysts on support materials are widely used in industry. Such catalysts are used to catalyse the combustion of a wide variety of gases, and are also used as the heat producing elements in heaters, small domestic appliances and the like. They are also useful in pollution control environments, where waste combustible gases must be disposed of. In typical applications, a noble metal catalyst such as platinum, palladium, iridium, or another group VIII noble metal, or a mixture of more than one metal, is placed on a finely divided support material. The finely divided support material is necessary so that the expensive noble metal will be exposed to as much of the gas flow as possible.
The finely divided support material must in turn be supported in some way. It can form part of a free standing porous composite material, such for example as disclosed in European published application number 0316 159 of Suppiah, published May 17, 1989. Alternately, it can be disposed on the surface of a massive ceramic material or extruded into a monolith. However, in both of these cases, heat travels from the reactions site of the combustible gas on the noble metal through the supporting structure. This tends to reduce the temperature of the noble metal catalyst, and may reduce it below the temperature where combustion may continue. Thus, it is often necessary to heat up the support, so that it will not withdraw heat and interrupt the combustion reaction.
It is therefore desirable to have light weight support materials with little thermal mass. For this reason, porous support materials are sometimes coated onto aluminum strips, and the noble metal catalyst is then applied to the support material. In at least one case, alumina has been formed on a screen, by coating the screen with aluminum hydroxide and then calcining the aluminum hydroxide to alumina. This gives rise to an alumina supporting material on a screen, which presumably has little thermal mass. See Japanese published application number 50-67294 of Eguchi. It has been known to attach alumina to an asbestos cloth, by spraying. See U.S. Pat. No. 3,198,240 of Keith. Painting has been disclosed to apply cobalt silicate to a screen for catalytic purposes. See British published application 2048945.
In recent years several highly crystalline forms of silica, which have zeolite type structures, have been developed. These will be described herein by the term "High Silica Zeolites". They have very high molar silica content, having a molar ratio of silica to alumina of greater than 10:1 and preferably greater than 20:1. They are intrinsically hydrophobic and remain stable at temperatures, in excess of 500.degree. C. One example of such high silica zeolite is silicalite, marketed by Union Carbide Corp., the preparation of which is described in U.S. Pat. No. 4,061,724 of Gosse. Other such zeolites are the ZSM series (ZSM-5, ZSM-8, ZSM-11 and ZSM-12) marketed by Mobil Oil Corp. These zeolites are disclosed in U.S. Pat. No. 3,702,866; U.K. patent 1,334,243; U.S. Pat. No. 3,707,979 and U.S. Pat. No. 3,842,449 respectively. Another high silica zeolite is called "ultrastabilized Y" and is described at American Chemical Society, Advances in Chemistry Series 101 266-278 (1971).
The hydrophobicity of high silica zeolites make them desirable materials for use as support for a noble metal catalyst. Also, they function alone as effective adsorbents for organic compounds. However, no techniques exist for applying high silica zeolites to the surface of a larger support particularly one of small thermal mass. High silica zeolites do not adhere in a permanent fashion to a metal screen or surface when painted or sprayed on, or when a screen surface is dipped into a high silica zeolite slurry.
According to the present invention, high silica zeolites can be attached to screens, strips or plates of glass fibre.
It has been known in some cases to use polytetrafluoroethylene (PTFE) in association with a high surface area catalyst support substrate. See for example U.S. Pat. No. 4,536,488 (Wanke). However, this has involved coating of a catalyst support material with PTFE. The binder PTFE tends to cover the pores of the support, therefore making it have less surface area, and hence not be as good a support material for noble metal catalyst. Although the present invention can in one embodiment make use of PTFE, much smaller amounts are used than in the Wanke process, and these amounts are preferably removed by calcining.